Lehan started two games this season, but has battled injuries and hurt his hamstring in practice Thursday.
"He was coming in and out of ballgames," coach Tony Sparano said. "We need consistency. You need healthy people."
Nose tackle Jason Ferguson practiced Friday for the first time since leaving last week's game against Baltimore with a rib injury.
Thomas played in four exhibition games for the Dolphins before being released Sept. 2. He was a third-round draft choice by Green Bay in 2004, but hasn't played in an NFL game since 2005.
The roster moves could mean more playing time against Buffalo on Sunday for reserve cornerback Jason Allen, the Dolphins' top draft pick in 2006. Thomas provides depth.
"It feels good to be back," Thomas said. "Whatever my role is, I'll do it to the best of my ability. If they ask me to hold the water, I'll be the best water holder."